1. Field of the Invention
The present invention relates to methods and apparatus for eliminating the need for color interpolation in single-array digital imaging systems by detecting modulated red, green and blue light at each pixel, and band pass filtering the detected light to extract signals representing red, green and blue.
2. Description of the Prior Art
In digital cameras, there are three generally accepted methods for acquiring color images using a light detector array. The first method is to put a color filter array (CFA) over the detector array, such that certain pixels detect certain colors. For example, some of the pixels would detect red light, some of the pixels would detect green light, and some of the pixels would detect blue light. The result of using such a color filter is that each pixel in the array sees only one color, so that to obtain a full color image, interpolation must be used to estimate the other two colors at each pixel location. FIGS. 1A and 1B (prior art) show how this method is used. The image obtained by the detector array through the color filter array has an arrangement of red pixels, blue pixels, and green pixels, for example in the Bayer CFA pattern, as shown in FIG. 1A. The intensity at each pixel indicates how much of the selected color of light was detected. In FIG. 1B all of the pixels have all three colors of light, because the green light is interpolated between pixel G1 and pixel G4 to determine the intensity of green light in the upper left hand pixel, and so on. Thus, each three color pixel is only really accurate with respect to one of the three colors. In addition, each color array is sparsely filled. This decreases the sample rate at each color and causes increased aliasing in the image.
The second method is more expensive, and more accurate with respect to color. Three separate detector arrays are used, one for each color. Thus three images, one for each color, are formed. Then the three images are overlapped to give a composite full color image. The grey scale resolution is no better than the first method, but the accuracy of the colors is greater. FIG. 2 (prior art) indicates how the three images are overlapped to generate an accurate three-color image. In addition, each array has a 100% fill factor (ratio of pixel size to pixel spacing), meaning there is little or no dead space between pixels. Consequently, aliasing is minimized.
The third method is to use a color filter wheel so that only one color illuminates the object at a time. This has the disadvantage that the field rate must be tripled to give the same frame rate. In addition, if the object moves during the exposure, the image acquires color artifacts. For example, a black/white edge appears as a color edge.
A need remains in the art for methods and apparatus for increasing resolution and color accuracy in digital imaging systems without the need for employing multiple detector arrays, by temporally modulating red, green and blue light, detecting all three colors at each pixel, and band pass filtering the detected light to extract signals related to red, green and blue at each pixel.
It is an object of the present invention to provide methods and apparatus for increasing resolution and color accuracy in digital imaging systems without the need for employing multiple detector arrays, by temporally modulating red, green and blue light, detecting all three colors at each pixel, and band pass filtering the detected light to extract values for red, green and blue at each pixel.
This is accomplished by sinusoidally modulating red, green and blue light sources at different frequencies. The modulation can be performed, for example, by modulating the current of the light source. The detector array detects all three colors of light at each pixel. A detector which preserves the modulation, such as a complementary metal oxide semiconductor detector is used. Three band pass filters, centered at the three modulating frequencies, are used to extract the signal associated with each color.
Alternatively, if an integrating detector array such as a charge coupled device (CCD) is used, the three illumination sources can be switched on and off sequentially, and the output read out for each color, at different times. The switching must be at three times the frame rate.
The present invention is useful in devices where artificial illumination is used, such as microscopes, endoscopes, color scanners, and manufacturing inspection cameras.